Chevrolet Repair: 1999 Suburban stumble/hesitation, throttle position sensor, intake manifold gasket

Question
Hi Van-I have a bit of an issue...my Suburban (1999 4wd 350 atm) used to get about 17mpg, now around 9mpg. With gas prices the way they are, I need to figure this out, as it is getting expensive...Had a steady miss on #5 cyl, replaced cap, rotor, plugs, wires. Miss fixed. However, I still seem to have a hesitation/stumble at 2400-2600 rpm, steady throttle. Seems low on power. Fuel pressure should be at 66lbs, mine is at 54...what do you think? Possible fuel pump? Reading remains same after blocking return lines.
Any input/thoughts?

Answer
Hi Jim,
I have seen the little hose inside the tank where the fuel pump connects to the line, get a split in it, and after reaching a certain pressure will open up and allow fuel to spray inside the tank.
I would be surprised if it caused that much difference in milage though.
A vacuum leak might cause it though.
Sticking or dirty EGR valve, intake manifold gasket, erc.
A bad throttle position sensor or mass airflow sensor could allow the wrong mixture without necessarily setting a code also. They can be tested.

Van